The present invention relates to electric lamp failure indicators and more particularly to a warning device for providing an indication that failure has occurred, e.g. in a lamp which fulfils an important function. The invention is especially, although not exclusively, applicable to front and rear lamps of motor vehicles to provide an indication to the driver, inside the vehicle, that such lamps are functioning correctly.
Although in the following description reference is made specifically to motor vehicles, the invention is also applicable to apparatus and instruments where one or more lamps are relied upon to provide important information and where it is desired to avoid unrecognized failure of such lamps.
Lamp failure indicators for use with front and rear lamps on motor vehicles have been proposed previously. In one system, a low value electrical resistor is connected in series with a lamp being monitored; the voltage drop across the resistor which results when the lamp is switched on and working is combined with corresponding signals from the other lamp circuits to hold off a warning lamp. If any one lamp fails, the warning lamp lights. The disadvantage of this system is that the presence of the resistor reduces the output available from each lamp and the need for an expensive circuit arrangement to detect the small signals from said resistor.
In another known system, a current balancing relay is employed. The current taken by each lamp being monitored flows through a solenoid, the solenoids being arranged so that the magnetic field produced by the current of one lamp of a pair opposes the field produced by the other lamp of the pair and thus a null field exists when all lamps are operating. If a lamp fails, the magnetic field is no longer balanced and a magnetic reed switch is caused to be closed to switch on a warning lamp. For this approach to be successful, there must be a reasonable close balance in the power taken by each lamp in a pair and this causes difficulties due to the wide tolerance in lamp ratings.
It is a purpose of the present invention to overcome or minimize one or more of the disadvantages of the above lamp failure indicators.
Both known systems described above can only indicate whether a lamp is operating correctly or has failed when the lamp is switched on, but it is desirable to have an indication that the lamp has failed even when it is switched off, e.g. during the hours of daylight, in the case of a motor vehicle, when it is most convenient to replace the lamp.